Chironomid Influence on Phosphorus Release in Sediment‐Water Microcosms

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Abstract
Microcosms of Lake Mendota, Wisconsin, USA sediment and oxygenated water, maintained in darkness, were used to evaluate phosphorus increments in the overlying water as influenced by 2 species of chironomids, chironomid density and chironomid—temperature interactions. Similar systems were held in light and tested with algae present to determine if the presence of chironomids enhances algal growth. Both species, Chironomous riparius and Chironomus tentans, cause increases in phosphorus content of overlying water, with the larger C. tentans producing the greater effect. Release of total phosphorus at 20°C increased approximately linearly from 0.3—9.4 mg·—2·—1 over a range of 6 densities from 0—6585 larvae/m2. Increasing temperature from 10—20°C had little effect on phosphorus release rates in microcosms without chironomids, but in systems with C. tentans increased temperature resulted in a °10—fold increase in total phosphorus release rates. Most phosphorus released to the overlying water was soluble reactive phosphorus (SRP) which was quickly taken up by algae in lighted microcosms. Lighted controls had little total phosphorus in the overlying water or on glass substrates (wall growth), while microcosms with C. tentans contained significantly more total phosphorus in the overlying water and on glass substrates. Excretion by chironomids, rather than translocation of phosphorus from the interstitial waters of the sediment, could explain the observed data. Regardless of the mechanism, chironomids enhance phosphorus release and where abundant in the aerobic regions of lake sediments, their activities may be expected to result in a significant source of phosphorus to the epilimnetic region.